Most cases of oral malodor originate in the oral cavity and not from the stomach. Within the oral cavity, an important source of malodor is along the gumline, particularly in the approximal regions between teeth and in periodontal pockets. These are sites where bacteria accumulate into plaques and biofilms. Bacteria ferment and metabolize food debris at these sites to generate volatile sulfur compounds (VSCs), thought to be the main component of oral malodor. The main VSCs found in human oral malodor include hydrogen sulfide, methyl mercaptan and dimethyl sulfide although other mercaptans (R-SH) and sulfides (R.sub.2 S) are found in oral malodor. Another important source of malodor is the surface of the tongue which at the microscopic level contains many recessed folds and cavities in which micro-organisms grow and flourish. Again, these are good sites for the production of VSCs. Impaired or reduced salivary flow also seems to correlate with increased oral malodor. Since oral micro-organisms are responsible for the VSCs originating in the oral cavity, the chronic use of antimicrobials in oral compositions has been found to improve oral malodor. Also, oral compositions containing zinc salts have been used to treat VSCs because they strongly complex with these sulfur compounds, thus rendering them non-volatile and no longer able to cause malodor.
Zinc salts have been used in oral compositions because they possess both antimicrobial and oral deodorizing properties when appropriately formulated. In particular, for zinc salts to be effective in an oral composition, they should be solubilized in a form that is either uncomplexed or weakly-complexed. Unfortunately, uncomplexed or weakly-complexed zinc has an unpleasant, astringent taste as well as a drying, sometimes metallic, aftertaste. Thus, high levels of uncomplexed (or weakly-complexed) zinc, though effective, result in compositions that can be too astringent to be generally accepted by consumers. Zinc chloride, for example, has been used in compositions as the source of zinc ions. However, in order for these solutions or rinse compositions to be stable, they are formulated at acidic pHs of approximately 3.0 or less. These compositions, as mentioned above, also, have an astringent taste and an unacceptable aftertaste.
Several methods have been used to ameliorate the negative aesthetics of oral zinc compositions. One method, suitable only for dentifrices or solid dosage forms, involves using insoluble or sparingly soluble zinc salts such as zinc citrate. These salts then dissolve in saliva when they are introduced into the oral cavity, thus supplying the requisite zinc ions needed for efficacy. A second method is to complex the zinc ion with a chelating ligand so that the level of uncomplexed zinc is reduced. A third method, again, only suitable for dentifrices or solid dosage forms, is to physically encapsulate zinc salts with hydrophobic polymers. The use of such polymers, thus, provides a delivery system which results in the extended release of zinc.
In order to produce stable mouthwashes, oral rinses or gels that are optically clear, the first and third methods cannot be used. As a result, chelants have been used to solubilize insoluble zinc salts or to form clear, stable solutions or gels above pH=3.0 (U.S. Pat. Nos. 4,289,754; 4,289,755; and 4,325,939, insoluble zinc citrate solubilized by excess citric acid and addition of sodium hydroxide to generate clear solutions with a pH of from 6.0 to 7.2).
Oral compositions have also been formulated with both zinc salts and CPC. U.S. Pat. Nos. 4,022,880 and 4,339,432 refer to the use of N-cetyl pyridinium chloride monohydrate (CPC) and zinc salts.